Augmented or mixed reality is a technology that allows virtual imagery to be mixed with a user's actual view of the real world. A see-through, near eye display device may be worn by a user to view the mixed imagery of virtual and real objects. The display device displays virtual imagery within a portion of the user's field-of-view. More specifically, virtual imagery is displayed within a see-through display region of the head mounted display device, which may include left and right see-through display regions for viewing by the left and right eyes of the user. While such a display region is see-through, the display region has optical characteristics, such as a transmittance, that affects (e.g., attenuates) ambient visible light that is incident on the display region. For example, the display region may have a 45 percent transmittance, meaning that only 45 percent of the ambient visible light intensity that is incident on the display region travels through the display region and is incident on the user's eyes. Another way of explaining this is that the display region may cause ambient visible light to be dimmed by 55 percent. If the display region does not occupy the user's entire field-of-view, this can cause a non-uniformity where some regions within the user's field-of-view will be darker than others.